The maintenance of polyoma virus in transformed syrian hamster cells is being studied. The site of integration of thermosensitive A gene mutants is being characterized by Southern blot hybridization; the expression of "early" gene products of the resident viral genome is being characterized by immunoprecipitation of 35S methionine-labeled proteins; a non-selective system that does not require expression of the "transformed phenotype" will be used to study maintenance and segregation of polyoma DNAs.